Selecting audio amplifiers for mobile devices

The recent rapid growth in smartphone and tablet usage in consumer electronics has driven a requirement for a much higher standard of audio performance than that of previous generations. This article aims to examine several areas of improving audio in mobile phone and portable audio systems - many of which use a processor with integrated analogue audio outputs.

Ever increasing consumer demand for an exceptional listening experience every time a user picks up their mobile phone or plugs in their headphones presents a number of technical and system level challenges which the audio architect has to overcome. It begins by examining a range of problems which can degrade the systems audio performance, presenting the designer with a number of challenges. As these challenges are presented, a range of potential solutions will be discussed.

Let us examine the end user's listening experience from two common use cases: listening to audio through headphones and listening to audio via the loudspeaker. Consumers now demand crystal clear voice calls in noisy environments; hi-fi quality audio playback for watching mobile video content; the ability to use their device as a conference phone in hands-free mode; and, of course, exceptional playback time for digital audio playback.

Despite these demands, the playback time is still often too short when using headphones and speakers for all content types, and the speaker response is sub-standard when using loudspeakers in hands-free mode. There is also the relentless quest to minimise PCB height and area, reduce the hardware bill of materials, and reduce software development time to meet demands for sleek, low-profile gadgets which can hit the market as fast as possible and also make the manufacturer a profit.

Headphones
Headphone amplifiers are not always ground-referenced, meaning that decoupling capacitors need to be charged to a reference level (VMID) on power up, and discharge to ground on power down (1). This can result in audible pops and also reduces bass response due to the capacitor forming a high pass filter with the headphone load impedance (50Hz in the case below). The additional expense of AC coupling capacitors is to be avoided, if possible.

Figure 1 – VMID referenced headphone amplifier outputs

Listening to headphones over a prolonged period of time will undoubtedly drain the battery; the challenge is to reduce the headphone amplifier's active power consumption as much as possible to extend the battery life. Using a highly-efficient class G headphone amplifier with ground-referenced outputs will not only extend battery life, but also remove the need for AC coupling capacitors (2), improve headphone bass response and minimise troublesome power-on/off pops and clicks. DC-offset removal at the headphone outputs can be achieved by using a circuit such as a DC servo, which measures output DC offset and automatically zeroes DC voltage (achieving sub 1mV) during device initialisation.

Figure 2 – Ground referenced headphone output

Class G headphone amplifiers will also extend battery life for headphone playback over traditional single supply class AB headphone amps, as often found on highly integrated SoC, due to adaptive voltage-rail switching and charge pump switching frequencies (3, 4). A charge pump-based approach to power headphone amplifiers is favoured over a buck converter-based design in order to keep the size of the external components down. The buck converter requires an inductor in the region of 4.7µH, which often has a larger PCB footprint than three 2.2µF capacitors required for a charge pump-based scheme.

The impact of Class G and switching-frequency thresholds as shown above can be illustrated by looking at quiescent current consumption of WM9090. Figure 5 is a sweep of quiescent current across PGA gain for a headphone playback path - in this case differential input on IN1+/- to stereo headphone. The plot includes an equivalent path for a competitor device employing similar charge pump architecture, but without class G or the clock scaling techniques adopted by Wolfson. Each threshold is clearly defined by a ‘step' reduction in current consumption, with performance of WM9090 much lower than the competitor device.

Wolfson's Class-G amplifiers switch the supply rail voltage to the amplifier by using a dual-mode charge-pump with variable frequency. This decreases power consumption and increases efficiency.
Wolfson's Class-W technology takes this a stage further, and is used in their CODECs, where a digital signal path is present. It includes control logic with predictive signal processing and adaptive dual-mode charge pump. This further reduces power consumption. It also improves audio quality because signal peaks are not clipped and does not introduce extra delays.